1. Field of the Invention
The present invention relates to a reference signal generation apparatus for a position detector of a positioning device.
2. Description of the Prior Art
Photoelectric encoders are widely used as position detectors of positioning devices. Such a photoelectric encoder comprises a moving slit plate adapted for rotary or linear motion and a fixed slit plate having a multiplicity of slits for generating counting signals and a reference signal, and operates to detect the position of an object by optically receiving pulse outputs from a plurality of light detectors. An example of such a conventional reference signal generation apparatus for a photoelectric encoder will be explained below with reference to the accompanying drawings. FIG. 10 shows an outline of a conventional photoelectric encoder. Reference numeral 1 designates a moving slit plate in rotary motion, numeral 2 a light source, numeral 3 a fixed slit plate, numerals 4, 5, 6 light detectors, and numeral 7 a rotary member. Numeral 8 designates slits for generating a counting signal which are formed on the moving slit plate 1. Numeral 9 designates slits for generating a counting signal which are formed on the fixed slit plate in opposing relation to the slits 8. Numerals 10, 11, 12 and 13 designate slits for generating reference signals, of which the slits 10, 11 are formed on the moving slit plate 1, and the slits 12, 13 on the fixed slit plate 3. The slits 10 and 12 are arranged in opposing relation to each other so that the light from the light source reaches the light detector 4 therethrough. The slits 11 and 13 are also arranged in opposing relation to each other so that the light that has passed therethrough is detected by the light detector 5. The slits 8, 9 and the light detector 6 are also arranged in similar manner. FIG. 11 is an enlarged view of the slits 10 to 13 for generating the reference signals, and FIG. 12 is a diagram for explaining the processing of an output signal of the light detectors 4, 5. Numeral 14 designates circuit means for producing a signal C making up the difference between a signal B produced from the light detector 4 and a signal A produced from the light detector 5. Numeral 15 designates circuit means for comparing the signal C with a threshold value D to produce a reference signal in pulse form when predetermined conditions are satisfied. FIG. 13 shows the signals plotted in a graph where the abscissa represents the lapse of time with the rotation of the moving slit plate 1 and the ordinate the values of the respective signals.
The operation of the photoelectric encoder configured as described above will be explained below. The moving slit plate 1 is adapted for rotating integrally with the rotary member 7. In this process, the light passing through the slits 8, 9 are interrupted, thereby producing a counting signal output from the light detector 6. The slits for generating the reference signals are arranged at random as shown in FIG. 11, so that a peak output is produced only when two corresponding slits are overlapped in registration with each other. Specifically, the slits 11 and 13 are arranged the same way, and the signal A with a peak P.sub.1 is produced from the light passing them. The slits 10, 12, on the other hand, are arranged with the light-interrupting portion and the light-transmitting portion thereof in opposing relation to each other for producing the signal B with a peak P.sub.2. These slits and the light detectors are arranged in such a manner as to produce the peaks P.sub.1 and P.sub.2 substantially at the same time. The signal C, which is taken as a difference between the signals A and B, offsets the variations of the signals A and B caused by the changes in light quantity or noise components carried in each of the signals thereby to improve the anti-noise ability. This difference signal C is compared with a threshold value D, and when the value of the difference signal C is larger than the threshold value D, the reference signal is produced. If the threshold value D is set to a level associated only with the peak P.sub.3 of the difference signal C but not with other peaks, it is possible to obtain a reference signal only at a position that is the position of the peak P.sub.3. Also, in view of the fact that the difference signal C takes a waveform similar to a triangle in the vicinity of the peak P.sub.3, the width of the reference signal is capable of being adjusted within a certain range by adjusting the value of the threshold D. Specifically, if the threshold value D is increased, the reference signal is reduced in width, while if the setting of the threshold value D is reduced, the width of the reference signal is increased. In the event that the threshold value D is set to an excessively large value, however, it becomes impossible to produce such a reference signal because of the decrease in the peak P.sub.3 of the difference signal C. If the setting of the threshold value D is too small, by contrast, peaks other than the peak P.sub.3 of the difference signal C which may have large amplitudes due to noise or the like exceed the threshold value D, thereby undesirably generating a reference signal associated with that particular position. As a result, values that are available as the threshold value D are limited to an appropriate range, and therefore the range of adjusting the width of the reference signal is also limited. Further, in the process of adjusting the width of a reference signal, the position thereof is also changed, thereby making it very difficult to adjust the position and width thereof independently of each other.
Explanation will be made now of another example of a conventional reference signal generation apparatus of a photoelectric encoder, which apparatus is different from the conventional apparatus described above. FIG. 14 shows a general configuration of such a conventional apparatus, in which the moving slit plate 1, the light source 2, and the fixed slit plate 3 are similar to those shown above respectively. Numerals 16 and 17 designate light detectors, numerals 18, 19, 20 slits, of which the slit 18 is formed in the moving slit plate 1, and the slits 19 and 20 in the fixed slit plate 3. The light detector 16 is in opposing relation to the slit 19, so that with the rotation of the moving slit plate 1, the slits 18 and 19 also come to be opposed thereto, with the result that the light from the light source 2 reaches the light detector 16 through the slits 18, 19. The light detector 17, and the slits 18, 20 are also constructed in similar manner. Each slit has a single opening, and the slits 19, 20 are formed in spaced relationship adjacent to each other. FIG. 15 is a diagram showing the processing of output signals of the light detectors 16 and 17. Numeral 21 designates circuit means for producing a signal G making up a difference signal between an output signal E of the light detector 16 and an output signal F of the light detector 17, and numeral 22 circuit means for producing a reference signal from the signal G. FIG. 16 shows a graph in which the abscissa represents the lapse of time with the rotation of the moving slit plate 1 and the ordinate the respective signals.
The operation of the reference signal generation apparatus configured as explained above will now be described. The light detectors 16 and 17 both receive and produce the light passed through the slit 18. As a result, the signals E and F which make up outputs of the light detectors 16 and 17 respectively reach a peak output value after the lapse of as much time as required for the slit 18 to move between the slits 19 and 20. A signal G having a point of zero between the positive and negative maximum values is obtained as a difference between the signals E and F. A method has been used by which a point where the value of the signal G becomes zero is detected, and a reference signal is produced during a very short period from the instant of the particular detection. In this method, in order to specify a point where the signal G becomes zero, the signal G is compared with the threshold values H and I and positive and negative peaks are detected to determine a point where the signal G becomes zero between the peaks.
This configuration, however, has a problem as described below. First, in the first conventional method first above described, if a reference signal corresponding to a high resolution is to be obtained, it is necessary to sharpen the peak P.sub.3 with a reduced width thereof to reduce the minimum width of the slit. At the same time, the interval between the moving slit plate and the fixed slit plate is required to be shortened. This construction, however, is actually difficult to realize in view of the problem of assembly, slit contamination or damage due to dust. Also, in order to produce an output like signal A or B, the slit 10 is required to include an opaque part in the light-transmitting part thereof. The light from the light-transmitting part is liable to circumvent to other light-receiving parts, often adversely affecting the counting signal. To prevent this inconvenience, the slit 10 is required to be located in spaced relationship from the counting slit, thereby making it difficult to reduce the size of the apparatus as a position detector. In the second conventional apparatus described above with slits having a single opening, on the other hand, less luminous energy is transmitted, resulting in a decreased output and inferior signal-to-noise ratio. A method to solve this problem in keeping with the trend toward higher accuracy involves reducing the slit width and increasing the opening area. Such a method, however, has its own limitation as the apparatus is increased in size. Also, since a reference signal is produced only for a very short period of time from a point, the position of the reference signal undergoes a change depending on the direction of rotation.